President Trump declared on Wednesday evening that he is “studying and reviewing the possible reduction of Troops in Germany,” in what appears to be retaliation for comments by Germany’s chancellor, Friedrich Merz, that Iran has “humiliated” the United States.

Even before Mr. Merz made his comments to a group of German students, the United States had hinted that it may review its troop levels in Europe, despite the continuing concerns that Russia’s president, Vladimir V. Putin, may attempt to test whether NATO would truly come to the aid of one of its smaller, newer members, like Estonia or Latvia.

Germany is the hub of the American presence in Europe, with about 35,000 American troops based there. It is also the center for medical treatment, aircraft arming and maintenance, and the headquarters for American forces in Europe. Italy houses 12,000 American troops, and the United Kingdom about 10,000, and all three have been critical way stations for forces and aircraft flying to the Middle East to support Operation Epic Fury, the attack the United States and Israel launched against Iran.

Military commanders, already feeling stretched, have been clear about the risks for the United States in cutting troops at the major bases across Europe. But for Mr. Trump, the decision may well hinge on his feelings about Mr. Merz, a former investment banker who has nurtured a relationship with him — but who has been increasingly critical about a war he has viewed as unwise and possibly illegal.

The blowup began when Mr. Merz spoke at a high school assembly in western Germany. “The Americans obviously have no strategy,” he said, “and the problem with such conflicts is always that you don’t just have to go in, you also have to get out again. We saw that very painfully in Afghanistan for 20 years. We saw that in Iraq.”

“So this situation is, as I said, at least ill-considered, and I do not see at the moment what strategic exit the Americans are choosing now.”

Germany, like other allies, was frozen out of the Trump administration’s deliberations about going into Iran, and then showed no enthusiasm for joining the fight, either individually or as part of NATO, the alliance created after World War II to contain the Soviet Union. Mr. Trump reacted with fury, suggesting anew that he was reconsidering America’s participation in the alliance, though he could not formally pull out without the approval of Congress.

He was particularly incensed by Mr. Merz’s comments and accused him of siding with Iran. “The Chancellor of Germany, Friedrich Merz, thinks it’s OK for Iran to have a Nuclear Weapon. He doesn’t know what he’s talking about!” Mr. Trump wrote in a social media post. For good measure, he added: “No wonder Germany is doing so poorly, both Economically, and otherwise!”

Mr. Trump did not say who was conducting the review, though he did say a decision would be “made over the next short period of time.” Sometimes he moves on to other grudges without following through on his threats; minutes after posting his threat to Germany, he attacked Jerome Powell, the outgoing chairman of the Federal Reserve, for saying that he would remain in his post as a member of the Fed even as his time as chairman ends. But his anger at NATO, and his particular focus on Mr. Merz, makes it likely there will be some move to express his displeasure.